1. Field of the Invention
The present invention relates to a magnetic garnet material and a Faraday rotator which utilizes the magneto-optical effect using the magnetic garnet material. It will be noted that the Faraday rotator according to the present invention is used, for example, in an optical isolator or an optical attenuator.
2. Description of the Related Art
In optical communication or optical application equipment using semiconductor laser, an optical isolator, an optical circulator or an optical attenuator is widely used. A Faraday rotator can be cited as one of the essential elements for these devices.
Although a single crystal of YIG (yttrium iron garnet) and a bismuth substituted rare earth iron garnet single crystal are known as the material for the Faraday rotator, the Faraday rotator using a single crystal film of rare earth garnet replaced with bismuth formed by a liquid phase epitaxial method (hereinafter referred to as an LPE method) is the mainstream device, at present.
For example, a type of bismuth substituted rare earth iron garnet in which a general chemical formula is (Bi.sub.x Re.sub.y Gd.sub.5-x-y) Fe.sub.5 O.sub.12, Re is Lu or Yb or both Lu and Yb, and is 0.5.ltoreq.x.ltoreq.1.3, 0.1.ltoreq.y.ltoreq.0.7, is disclosed in the publication of Japanese Laid Open Patent Application No. 63-69718. Further, the type of bismuth substituted rare earth iron garnet in which a general chemical formula is Gd.sub.3-x Bi.sub.x Fe.sub.5-y-z Ga.sub.y Al.sub.z O.sub.12 (here, 0.90.ltoreq.x.ltoreq.1.05, 0.50.ltoreq.y.ltoreq.+z .ltoreq.0.65, 0.20 .ltoreq.y/z.ltoreq.0.27) is disclosed in the publication of Japanese Laid Open Patent Application No. 9-33870. Furthermore, recently miniaturization of the device for the optical isolator and the like and the optical attenuator using a magnetic optical element as disclosed in the publication of Japanese Laid Open Patent Application No. 9-236784 are focused, thereby increasing the requirement of the Faraday rotator which saturates in a low magnetic field.
The Faraday rotator which saturates in the low magnetic field can be easily obtained by replacing an Fe element with a non-magnetic element such as Ga, Al, etc. and, in fact, some have been proposed.
However, when the Fe element is replaced with a non-magnetic element, a Faraday rotational capacity reduces, thereby resulting in a defect that the thickness of the element must be made thick. The thickness of the element for the Faraday rotator in which the magnetic field required for saturation is less than 200 (Oe) needs to be approximately 500 .mu.m in the case of the Faraday rotator for the optical isolator working at the wavelength of 1550 nm, and 1000-1200 .mu.m for the optical attenuator working at the wavelength of 1550 nm. Further, when an amount of the non-magnetic element increases, a magnetic compensation temperature becomes above 0.degree. C., thereby resulting in a problem that a temperature range for using the Faraday rotator is limited.
When forming the single crystal film of magnetic garnet according to the LPE method, cracks easily occur during film growth because heat expansion coefficients for a substrate material such as a single crystal of gadolinium-gallium-garnet (hereinafter referred to as GGG) with additives of Ca, Zr and Mg and for a single crystal film of magnetic garnet differ by approximately 20%. Although the tendency becomes particularly prominent when the thickness is more than 500 .mu.m and efforts to avoid the cracks are made by various methods, reducing the thickness of the element is one of the most effective methods.
Since in the LPE method, a solid phase is separated from a liquid phase in a supersaturated state to a substrate for epitaxial growth, the LPE method always contains a possibility to separate the solid phase other than an epitaxial film. When such a solid phase is separated, a problem of crack occurring to a surface of the epitaxial film or a considerable reduction in growth rate is caused. Since when the thickness of the film exceeds 500 .mu.m, the time to be exposed in the supersaturated state reaches several tens of hours, this problem is easily produced.
Further, when the Faraday rotator is obtained from the formed single crystal film of magnetic garnet, a film thicker more than approximately 100 .mu.m than the thickness of the element is required for performing a substrate elimination or surface polishing. Therefore, the above problems become more and more prominent.
Thus, when the thickness of the element becomes thick, not only miniaturization of the device is exerted an evil influence, but also cracks at formation of the single crystal film, an occurrence of a surface defect and remarkable reduction in growth rate are produced, thereby causing a reduction in yield, an increase in cost, and a reduction in productivity. Also, a situation arises to require a plurality of materials to make one rotator, thereby further increasing the cost.
For example, in the case of the rotator for the optical attenuator, wavelength is 1550 nm and the thickness of the element is equal to approximately 1-1.2 mm. Therefore, the single crystal film of magnetic garnet is required to be approximately 1.1-1.3 mm in thickness. Three pieces must be layered to be used as the material, thereby causing a problem of an increase in cost and complexity in handling.